A variety of capacitors are known for providing adjustable capacitance in electronic circuits. Conventional trimmable capacitors include a stationary conductor electrode, or stator, and a positionally adjustable conductor electrode separated from the stationary electrode by a dielectric, e.g., air. The capacitance is adjusted, or “trimmed,” by moving the adjustable electrode closer to or further from the stationary electrode. Often such capacitors are mounted on printed circuit boards by soldering the external terminal connections of the capacitor to conductive elements on the printed circuit board.
All capacitors are imperfect to some degree. That is, parasitic impedance is associated with the capacitor elements, particularly the conductive terminal leads. A particular problem that occurs at high power levels and ultra high frequencies approaching 1 GHz and beyond is increased power losses. The skin effect caused by the conductor through which current is flowing can exaggerate both the capacitor reactance and overall capacitor performance at high power levels and high frequencies. Thus capacitors demonstrating low power loss at low and moderate frequencies and power, may become increasingly lossy due to conductor losses as frequency and power rise significantly.
One well-known trimmer capacitor is of the so-called barrel type, to which the instant invention applies. In that capacitor, shown in FIGS. 1 and 2, the movable electrode is situated inside a tubular housing or casing, generally cylindrical in form. Both electrode terminals are stationary and located on the capacitor casing. One of the electrodes, the stator, is located at one end of an insulating section of the casing and is ordinarily for most applications directly soldered to a printed circuit board terminal. The internal circumferential surface of a conductive casing section is threaded to receive a threaded adjustable electrode that can be screwed toward and away from the stationary end electrode for “trimming,” or “tuning,” the capacitance value. The conductive section of the casing forms the other electrode terminal and is in intimate electrical contact with the movable electrode.
With the barrel-type configuration, the end stator electrode can be connected directly to the circuit in which it is used. For the other electrode, however, it is necessary to provide an external conductor to connect the terminal at the side of the casing to the circuit. It has been found, however, that at highly elevated frequencies, e.g., >1 GHz, the capacitor Q value is diminished to an undesirable degree, and this is due at least in part to the added non-capacitive impedance of the external terminal connection under those conditions. The challenge has been to find an efficient and economic solution to this problem in order to provide trimmer capacitor for use at frequencies in the range of 1–10 GHz and beyond, especially during relatively high power handling.